1. Field of the Invention
The present invention generally relates to cannula and similar hollow needle-like devices. More particularly, this invention relates to a method of forming miniature needle-like devices from single-crystal silicon-based material using micromachining and wafer bonding techniques.
2. Description of the Related Art
Medical delivery of drugs has been accomplished for many years using cannula and hollow needles. Metal needles have been miniaturized to very small sizes and integrated with attachments to increase functionality. However, the extent to which metal needles can be miniaturized is limited by processing limitations and the ductility of metals, the latter of which renders metal needles with small diameters prone to bending. In contrast, cannula and needles formed of silicon and silicon-based alloys such as SiGe and SiGeB are not ductile at room temperature and can be micromachined to a much smaller size, typically less than 100 micrometers in diameter, resulting in what is termed herein a microneedle. Because of this capability for greater miniaturization, there is considerable interest in fabricating cannula and other needle-like devices from silicon-based materials.
Silicon microneedles have typically been formed by a combination of micromachining and deposited layers. For example, U.S. Pat. No. 5,855,801 to Lin et al. discloses a process of forming microneedle a by wet anisotropic etching single-crystal silicon and depositing silicon nitride to define a microchannel within the microneedle. U.S. Pat. No. 5,928,207 to Pisano et al. discloses a process by which a silicon microneedle is fabricated by wet isotropic etching single-crystal silicon and depositing polysilicon. Another process described in K. Papageorgiou et al., “A Shuttered Probe with In-Line Flowmeters for Chronic In-Vivo Drug Delivery” combines reactive ion etching (RIE) a pattern of diagonal openings in the surface of a silicon substrate to define a grating, undercutting the grating by anisotropic etching to define a microchannel beneath the grating, and then sealing the openings of the grating with deposited films of silicon oxide, silicon nitride or polysilicon.”
A drawback to the use of deposited films of silicon oxide, silicon nitride, polysilicon, etc., on a single-crystal silicon micromachined features is the stress that results from grain size variation within deposited films and differences in coefficients of thermal expansion between the deposited films an single-crystal silicon. Such stresses increase the risk of bowing, warping and cracking of the micromachined features, which can lead to mechanical problems and high scrappage rates in the case of cannula and other types of microneedles. Deposited films also limit the wall thickness and internal cross-sectional area of microneedles, thereby limiting the degree to which a microneedle can be miniaturized.